1. Field of the Invention
The present invention relates to a fire alarm system in which one or a plurality of zone lines having a plurality of fire detectors connected thereto are connected to a receiver unit, such as a fire control panel or a repeater(s) connected to the fire control panel, via a signal line, the receiver unit receiving fire signals sent from the fire detectors.
2. Description of the Prior Art
It is known in the art of fire alarm systems that zone lines constituted of a pair of power supply/signal lines, etc. are connected to a fire control panel or to a repeater(s) connected to the fire alarm panel, and extend therefrom over zones, these zone lines having a plurality of fire detectors connected thereto.
Various types of fire detectors, such as a fixed temperature detector in which a mechanical contact is closed by the contrarotation of a bimetal, heat detector for detecting an increased heat rate in which a mechanical contact is closed by the expansion of a diaphragm forming an air chamber, and an ionization type, a photoelectric type or any other type smoke detector having semiconductor components such as an FET, amplifier circuit, etc., and having a switching circuit that initiates a self hold in response to the operation thereof, etc., are connected to the zone lines according to their setting positions. Each zone line includes various types of fire detectors as described above connected thereto. Some of the detectors described above may have response lamps that light in response to the detection of a fire indicating that the detector is operating, while the remaining detectors may not have such lamps.
On the other hand, a supervising power source in the fire control panel or the repeater is connected to the fire detectors via each zone line. When the fire control panel or the repeater receives a fire signal via a zone line from a fire detector which has operated in response to detecting a fire, it supplies any of the operated fire detectors via that zone line with current required to light the response lamp of the operated fire detector or required to cause the operation of the fire detector to be self-held.
As there are many types of detectors connected to one zone line as noted above, the fire control panel or the repeater evenly supplies those fire detectors with current from the supervising power source as well as current for the response lamp and for self-holding. Accordingly, even if fire detectors having, for example, only a mechanical contact closed by a bimetal, are connected to the zone line, the fire control panel or the repeater still supplies that zone line with current from the supervising power source and the lamp and self-holding current.
In the prior art, current of tens of milliamperes per detector is required to light a response lamp in a fire detector that is actuated or operated. If a plurality of detectors are operated together, a problem arises in that the large total current consumed by their response lamps requires a correspondingly larger capacity for an emergency power source such as a battery.
When a fixed temperature detector having only a mechanical contact but no response lamp or self-holding circuit is operated, a current flows through the closed mechanical contact of the fixed temperature detector in question to the zone line in order to transmit the fire signal to the receiver unit such as a fire control panel or a repeater. After the operation, the mechanical contact maintains its closed condition and the current continues to flow through the mechanical contact. However, no current flow through the mechanical contact and the zone line to the receiver unit is needed because it is sufficient to transmit only one fire signal to the receiver unit to inform the receiver unit of the fire situation, and accordingly, current subsequently flowing through the mechanical contact is ineffective. Moreover, because the resistance of the mechanical contact closed is small, the current flowing through the overall circuit from that mechanical contact to the receiver unit such as a fire control panel is decided only by way of a line resistance of the zone line and thus the ineffective current value is very large.
In order to prevent this ineffective current flow, it may be thought to provide two individual circuits or lines for each zone, one circuit for connecting fire detectors of a type having only a mechanical contact and the other circuit for connecting the other types of fire detectors which require the supply of current after operation. However, providing two circuits or lines in a single fire monitoring area increases the number of zone lines sharply and causes the system to be complex and expensive, whereby additional problems arise in that fire detectors might possibly be installed in the wrong circuit when alteration of an installed system is required.